Such a process is known from EP-A-776959. This publication describes a process wherein the high boiling fraction of a Fischer-Tropsch synthesis product is first hydroisomerised in the presence of a silica/alumina supported Pd/Pt catalyst. The isomerised product having a content of non-cyclic iso-paraffins of more than 80 wt % is subsequently subjected to a pour point reducing step. The disclosed pour point reducing step in one of the examples is a catalytic dewaxing step performed in the presence of a silica-supported dealuminated ZSM-23 catalyst at 310° C.
A disadvantage of such a process is that only one grade of base oils is prepared. A next disadvantage is that the hydrosiomerisation step is performed on a narrow boiling range fraction of a Fischer-Tropsch synthesis product, which hydroisomersation step is especially directed to prepare a base oil precursor fraction having the desired properties. The hydroisomerisation process step can also yield valuable large volumes of middle distillates next to base oil precursor fractions if the feed would also include more lower boiling compounds. There is thus a desire to prepare base oils from a waxy paraffinic fraction as obtainable from a hydro-isomerisation process step, which yields both middle distillates, such as naphtha, kerosine and gas oil, and the waxy paraffinic fraction having a content of non-cyclic paraffins of more than 80 wt %. There is also a desire to have a flexible process wherein two or more base oils having different viscosity properties are obtained of excellent quality.